A steering rack having two spaced apart toothed portions for use in motor vehicle electric power steering systems is known. A first toothed portion meshes with a pinion that is axially connected to the steering wheel of the motor vehicle whilst the second toothed portion meshes with a pinion that is connected to an electric motor. Such a steering rack is hereinafter referred to as a “dual pinion rack”. The first pinion may be axially connected to the steering wheel of the vehicle via a torque sensor. The torque so sensed is used to modulate the torque applied to the steering rack via the second pinion, thereby providing power assisted steering.
The majority of vehicle electric power steering systems in use today are column assist. The motor drive torque is imparted to the steering gear input shaft via a geared drive and the full driving torque required to steer the vehicle is delivered to the toothed portion of the steering rack via the pinion which is axially connected to the input shaft. The motor consumes space in the passenger cabin and can be an unwanted source of noise and heat. Hence there is increasing interest in design solutions that remove the electric motor from the vehicle cabin. The dual pinion rack earlier described is one such solution.
The known method of manufacturing dual pinion racks is to machine both the first and second toothed portions onto the rack bar.
There are disadvantages associated with machining toothed portions onto a rack bar, one of which is the wastage of material. Also, machining of toothed portions is not suitable for producing variable ratio tooth forms on rack bars and is therefore limited to constant ratio tooth forms.
However, forging methods similar to those described in U.S. Pat. No. 4,116,085 (Bishop et. al), U.S. Pat. No. 4,715,210 (Bishop et al,) U.S. Pat. No. 4,571,982 (Bishop et al) and U.S. Pat. No. 5,862,701 (Bishop at al) are suitable for forging both variable-ratio and constant ratio tooth forms. Certain advantages associated with forging racks are stated in these patents.
It has been considered to manufacture a first toothed portion on a dual pinion rack using these known forging methods, and subsequently manufacture the second toothed portion by a machining process. However, a drawback still remains that the machined toothed portion wastes material and is limited to constant ratio tooth forms.
Manufacture of variable and constant ratio steering gears by the methods and apparatus described in the aforementioned patents is well established with the majority of applications being in hydraulically powered steering gears. However, commensurate with the trend to electric power steering, forged variable ratio steering racks are being increasingly used. In addition to being able to choose an optimum on-centre ratio for good dynamic response, the designer may also choose to vary the mechanical ratio near the full lock part of the rack travel to provide an increased mechanical advantage for the electric motor.
The present invention provides a method that allows both the first and second toothed portions of a dual pinion rack to be manufactured by forging.
The present invention provides an apparatus which allows for the second toothed portion of a dual pinion rack to be manufactured by forging, and more particularly both the first and second toothed portions of a dual pinion rack to be manufactured by forging.